Chemistry

If a liter of a buffer is prepared with a final concentration of .3M monosodium phosphate and .5M disodium phosphate, what is the pH of this buffer? (Hint: Which is conjugate acid (HA) and conjugate base (A-)? What's the pKa for this particular pair of HA and A-?)

So I think the conjugate base HA is disodium phosphate and the conjugate acid A- is monosodium phosphate. Otherwise I'm not sure where to start and I'm not sure why or how I determine that at this point.

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asked by Shaun
  1. You want to use the Henderson-Hasselbalch equation. Substitute into
    pH = pKa + log[(base)/(acid)].
    The base is Na2HPO4. The acid is NaH2PO4.

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    posted by DrBob222
  2. Okay, so based on the chart in my book I was able to determine the following.

    pH=(unknown)
    pKa=7.2 of Phosphoric Acid (2)
    A-=.5M of Na2HPO4
    HA=.3M H2PO4-
    pH= 7.2 + log(.5/.3)
    pH=7.2+log(1.6667)
    pH=7.2+0.5108

    So, the pH of this solution is:
    pH=7.7108

    Correct?

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    posted by Shaun
  3. pH=(unknown)
    pKa=7.2 of Phosphoric Acid (2)
    A-=.5M of Na2HPO4
    HA=.3M H2PO4-
    pH= 7.2 + log(.5/.3)
    pH=7.2+log(1.6667)
    You are ok to here
    pH=7.2+0.5108
    I find 0.22 for the log of 1.6667

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    posted by DrBob222
  4. Upon recalculating this you are definitely correct...log(1.6667)=0.22
    so 7.2+.22=7.42
    and the pH is then 7.42
    Thanks again for your help.

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    posted by Shaun

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